1. Field of the Invention
The present invention relates to an image forming apparatus and more specifically relates to an image forming apparatus including a fixing unit that separates a sheet from a heating roller by a separation claw.
2. Description of the Related Art
Conventionally, as an image forming apparatus such as a copying machine, a printer, or a facsimile, there is an image forming apparatus forming an image by an electrophotography technique. In a case where an image is formed on a sheet by the image forming apparatus, a toner image is first formed on an image bearing member. The toner image is thereafter fixed permanently on a sheet to form an image on the sheet by toner.
The most effective method to fix the toner image permanently on the sheet is fusing powdered toner by applying heat and applying pressure so as to compress the toner and the sheet. Thus, the image forming apparatus using the electrophotographic process generally includes a fixing unit having a heating roller and a pressurizing roller as a means for fixing the toner image permanently on the sheet. In a case of fixing the toner image, the sheet to which the toner image has been transferred is made to enter a fixing nip portion formed by the heating roller and the pressurizing roller to heat, press, and fix the toner image on the sheet.
In the fixing unit, there is a case in which the sheet winds around the heating roller at the time of fixing. In order to prevent the sheet from winding around the heating roller, a configuration in which a separation claw is made to contact with the heating roller to separate the sheet from the heating roller by the separation claw is widely known.
At the time of fixing the toner image on the sheet, the toner on the sheet is offset to the heating roller. The separation claw abuts on the heating roller in a static state at all times. Thus, the toner is stripped off by the separation claw that abuts on the heating roller in a static state at all times and thereafter accumulates at the tip end portion of the separation claw. The toner accumulated at the separation claw is eventually hardened like a blade edge formed in a sharp wave shape since it accumulates unevenly. The hardened toner accelerates partial abrasion of the heating roller or scratches the heating roller.
Conventionally, as a means for solving this problem, a configuration of reciprocating the separation claw continuously or intermittently at constant speed in an axial direction of the heating roller while making the separation claw abut on the heating roller is proposed. This technique is disclosed in Japanese Patent Application Laid-Open No. H02-129680.
Also, conventionally, the heating roller and the pressurizing roller are held between frames provided in the fixing unit and cannot move in a width direction perpendicular to a sheet conveying direction, that is, an axial direction of each roller. Accordingly, when the sheet passes through the fixing nip portion, the end portion of the sheet passes the same positions of the heating roller and the pressurizing roller. Due to the contact with the sheet end portion at the time of passing, the heating roller and the pressurizing roller are abraded to shorten their longevity. As a means for solving this problem, a configuration of reciprocating the heating roller and the pressurizing roller continuously or intermittently at constant speed in their axial directions is proposed. This technique is disclosed in Japanese Patent Application Laid-Open No. 2005-351939.
In a case of reciprocating the heating roller and the pressurizing roller, the separation claw is held on the image forming apparatus main body. When the heating roller and the pressurizing roller reciprocate, the separation claw relatively reciprocates continuously or intermittently at constant speed in an axial direction of the heating roller while abutting on the heating roller. This prevents the heating roller from being partially abraded or scratched by the separation claw.
In a conventional image forming apparatus, when improvement in longevity of the rollers is attempted, the moving amount(s) of the separation claw or the heating roller and the pressurizing roller must be increased. In a case where the moving amount of the separation claw or the like is increased, the position of the separation claw, which should originally be sufficiently away from the sheet end portion, may be overlapped with the position of the sheet end portion (corner portion of the tip end). In a case where the position of the separation claw is overlapped with the position of the sheet end portion, the separation claw may cause corner folding of the sheet or sheet jam.
The present invention is accomplished by taking these circumstances into consideration thereof, and an object thereof is to provide an image forming apparatus improving longevity of a heating roller and enabling to prevent generation of corner folding and jam of a sheet.